Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NOx formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NOx emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NOx emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NOx is very sensitive to the ammonia content in the producer gas. Results show that although NO-NO2 interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NOx reduction. The results indicate that NOx can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NOx burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NOx is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.